Attribute expressions

closes #659
I'm not sure that I understand.  The current codebase does not believe that a `Column` is "like a series".  instead the `iscolumn` property determines this.  

I agree that handling of this requires care.  I'm not trying to make a statement about Blaze's handling of list-of-list vs list-of-values, simply a statement about the meaning of `Column` as is commonly used throughout the codebase (at least the parts that I wrote).